The present invention relates to a method and device suited to obtain a map (bi-dimensional graphic representation) of the extension and the amount of detachments (separation zones) of frescos and generally of mural paintings, in order to estimate the damage caused by environmental conditions (e.g. temperature fluctuations, humidity) to such works of art. Specifically, the present invention concerns a noninvasive or nondestructive method and device for analysing the separation zones, that is, such as to avoid damage to a work of art.
The research related to the detection and determination of the amount of detachment of the separation zones in frescos is one of the most urgent and difficult problems in the field of conservation and restoration of works of art (see Danti C., Matteini M., Moles, Le pitture murali, Tecniche, Problemi, Conservazione. Centro di Edit., Firenze, 1990). The need to deal with such a problem in a systematic and scientifically valid way is felt even more if we consider that the presence of frescos in the Italian artistic patrimony is very remarkable, both having regard to the number of works and to the value of these masterpieces which are unique in their kind. To better define the problem we are dealing with, it is useful to briefly recall the kind of process which was used to make an antique fresco. On a layer,of raw plaster named xe2x80x9carriccioxe2x80x9d (floating coat), there was applied a thinner layer of plaster, on which the artist sketched his drawing, and at last, a final layer of plaster (xe2x80x9cintonachinoxe2x80x9d or xe2x80x9cplaster finishxe2x80x9d) was applied, which was made of very thin mortar, and the latter was painted while it was still fresh. By the word xe2x80x9cseparation zonexe2x80x9d or xe2x80x9cdetachmentxe2x80x9d in a fresco, we mean the lack of adherence of the floating coat to the underlying wall structure and/or to the plaster finish (xe2x80x9cintonachinoxe2x80x9d), which gives rise, due to the permeability of the material, to air gaps whose shapes and dimensions vary considerably. In most cases, the reason for this is an excessive air humidity in the surroundings of the fresco. In the absence of adequate restoration work, fissures and complete separation of parts of the painted surface may occur. The technique most used by restorers, for the detection of separation zones in frescos, is an empiric and very invasive (destructive) one, called xe2x80x9chammerxe2x80x9d technique, which amounts to hitting the fresco surface or wall structure surface while listening to the difference in the sound emitted by the same.
At an international level, the problems mentioned above concern the more general patrimony of mural paintings, which includes also works of art which are not antique.
Even if this problem seems not to be as important as in Italy, there are however European countries like France, Germany, Austria and Greece, in which the need to effect precise diagnoses on the frescos is as urgent as in Italy. Among nondestructive scientific techniques, the only one which currently is suited to give some information on the presence of separation zones in frescos, is the so-called xe2x80x9cthermovisionxe2x80x9d (see Segal, Y., et al., Research Techniques in Nondestructive testing, Edit. R. S. Sharpe, Acad. Press, New York, 1977), which, however, often gives results which are difficult to construe. Moreover, its complex instruments is which are not easy to use and its very high cost, considerably limit the use of this technique.
Attempts aiming at providing -in this field- alternative techniques which could replace the thermovision, and in particular acoustic techniques, have not reached till now satisfying results. An example of these attempts is given by the xe2x80x9cecospectrographicxe2x80x9d technique first presented in xe2x80x9cUn progetto per Piero della Francescaxe2x80x9d (Matteini M., et al., 1989, Indagini diagnostico-conoscitive per la conservazione della Leggenda della Vera Croce e della Madonna del Parto, Alinari, Firenze), but no positive results are known in this respect, neither for applications concerning specimens nor for those of real frescos.
A further and more recent technique, based on a vibrometer system measuring the frequency response function xe2x80x9cacceleration/forcexe2x80x9d or inertance, seems to have reached some useful result on appropriate specimens under ideal laboratory isolation conditions, but experiments on real frescos have not yet been done (see Mannaioli A., 1992, Progetto e realizzazione di un sistema vibrometrico per l""identificazione di distacchi negli affreschi, Tesi di laurea, Fac. Ingegneria, Università xe2x80x9cLa Sapienzaxe2x80x9d, Roina, 1991/1992). Actually, the two above mentioned acoustic techniques, leaving out of consideration the laboratory results which have been thereby obtained, have the serious limitation that they cannot be classified as nondestructive.
In fact, they require that the painting be physically xe2x80x9chitxe2x80x9d by means of a mechanical source, in order to excite the structure to be analysed, and be xe2x80x9ctouchedxe2x80x9d by means of sensors responsible for the detection of the acoustic response.
For what concerns the technique employing a laser vibrometer, it has been noted that the same is very useful for studying the vibration modes of structures, due to the fact that it is totally independent from environmental noise, however, the actual research progress does not allow to use the sole modal analysis in order to discriminate in a univocal way the resonances of the separation zones from all possible vibrational modes of the excited structures comprising said separation zones.
The acoustic) technique suggested by the inventors is different from the above mentioned ones, with respect to the use of a different acoustic parameter (the acoustic energy absorption coefficient) as physical indicator of the separation zones, and moreover, with respect to a perfectly nondestructive method of analysis of the painting.
In fact, the acoustic source used to excite the surface, and the sensor detecting the acoustic signal reflected by the same, are both positioned at an apropriate distance from the fresco, without need to xe2x80x9ctouchxe2x80x9d it, while using acoustic waves of limited amplitude. The new proposed method utilizes an appropriate signal processing system allowing to discriminate the feeble signals indicating the detachment zone, from the other noise.